1. Field of the Invention
The present invention relates to a revolver developing apparatus for use in a multi-color image forming apparatus such as a copying machine, a facsimile, a printer, or the like.
2. Description of the Related Art
An image forming apparatus such as a full color copier is known to form color toner images on a transfer sheet by the following process steps: electrostatically forming a latent image of a color separated image on an image bearing member; developing each latent image with a toner of respective complementary colors; and transferring each color toner image to a single transfer sheet with each color toner image superimposed with each other. A multi-color image forming apparatus is also known which forms a latent image to be developed in each different color one by one on an image bearing member, develops the latent image with each developer of different color, and transfers each color toner image to a single transfer sheet with each color toner image superimposed on each other.
In the above-described image forming apparatuses, a developing apparatus with a plurality of developing devices is necessary to form multi-color toner images. Consequently, if respective developing devices are independently arranged around the image bearing member, then the size of the developing apparatus increases.
Accordingly, a revolver type developing apparatus has been developed. Specifically, this type of developing apparatus has a rotating member which supports a plurality of developing devices at predetermined positions thereof. The rotating member is rotated to locate sequentially the developing device at a developing position. Latent images formed on the image bearing member are developed by each developing device with a toner of a particular color.
In the above-described revolver type developing apparatus, it is necessary to provide a toner storing section of the developing apparatus which stores sufficient amounts of toner required for a predetermined number of image forming operations. A sufficient amount of toner must be supplied from the toner storing section to a developing roller to develop latent images on the image bearing member.
For example, Japanese Laid-open Patent Publication No. 58-172660 describes a developing device in which a plurality of toner storing sections are provided and toner is deposited in each of toner storing sections divided by radial partition members.
In the developing device described in Japanese Laid-open Patent Publication No. 58-172660, toner can not be replenished from outside of the developing device into the toner storing section. Therefore, when toner is consumed in any one of the toner storing sections, it is necessary to replace a whole developing device with a new one even though toner is left in other toner storing sections. Alternatively, if the capacity of the toner storing section is increased in order to reduce the frequency of exchange of the developing device and to extend the useable period of the developing device, the developing device becomes large.
Another developing device is known wherein the developing device includes a casing disposed below and facing an image bearing member and is formed with an opening facing the image bearing member; a developing roller accommodated in the casing and partly exposed through the opening of the casing; a supply roller having a longitudinal access for supplying a toner to the developing roller, a toner storing section provided at one side of the developing device for storing the toner; and a conveyor member located upstream of a position where the toner is to be transferred from the supply roller to the developing roller with respect to an intended direction of movement of the supply roller for conveying the toner in a longitudinal direction of the supply roller, wherein toner is conveyed from the toner storing section into the casing and deposited within the casing such that substantially all of the developer is deposited above the longitudinal axis of the supply roller.
In the above-described developing device, toner around the conveyor member is subjected to stress (e.g., pressure) due to rotations of the conveyor member. In order to improve the fluidity of the toner and to gain appropriate chargeability of the toner, additives having smaller particles and higher hardness than toner, such as silica, titanium oxide or other similar additives, are mixed with the toner. Consequently, when the toner is subjected to stress over time, additive particles in the toner fall among the toner particles, the shape of pulverized toner particle change to round, and the average toner particle consequently becomes smaller. As a result, the chargeability of the toner is changed, and thus the image quality deteriorates. In addition, the fluidity of the toner is changed, and thus the uniformity of the image is reduced.
When the developing device is rotated for a multi-color image forming operation and the opening of the casing is directed downward, the toner deposited in the casing may leak from the opening and scatter around out of the casing.